In order to form single screw compressors, or positive displacement type machines for varying the pressure to a gas, it is known to make use of combinations comprising a mainrotor, having a toroidal surface and projecting threads having a generally helicoidal shape. The crests of said threads are intended to cooperate with a casing, thereby forming compression chambers and the mainrotor is adapted to cooperate with one or a number of gaterotors, the teeth of which are in meshing relation with the threads formed on the mainrotor. Examples of single screw compressors are shown in U.S. Pat. Nos. to Zimmern 3,551,082, 3,133,695, 3,181,296 and 3,752,606.
The space formed between two adjacent threads of a mainrotor of this type cooperating with an internal surface of the casing can accordingly form a compression chamber which is sealed off at one end by a tooth of one of the gaterotors and sealed off at the other end by providing the casing with a closed end.
When a fluid such as air or gas enters into a compression chamber of this type, the rotation of the mainrotor permits a progressive reduction in volume of the compression chamber, compressing the fluid until said compression chamber is put into communication with an outlet which can be formed in the casing.
Because there is relative motion between the parts in the single screw compressor, the clearance between the parts can only be reduced to a minimum finite value. Even when clearances have been reduced to operational minimums, there are still a large number of paths where the fluid being compressed can leak out.
Current practice in the design and manufacture of single screw compressors is to rotationally support the mainrotor and gaterotors in a structure that is also a pressure vessel for containing the inlet pressure when the inlet pressure is above atmospheric pressure. To a large extent, the structure necessary for maintaining the axes in alignment and the structure necessary for containing the pressure of a pressurized inlet are interactive and must be compromised. In these machines, the casing not only provides the structural support for the mechanism but it also provides the containment of the inlet pressure. When the mechanism is operating at inlet pressures above atmospheric as is done when the mechanism is being used as a second or subsequent stage compressor, frequently in the range of 100 to 200 bar (1500 to 3000 psi), distortion of the pressure vessel results due to elastic deformation. This distortion will move the mainrotor and gaterotor rotational axes away from their desired alignment. Because of this shift in the rotational axes due to the elastic deformation of the supporting structure of a conventional machine, design clearances between the meshing rotors must be compromised and an unacceptable amount of the fluid being compressed is allowed to leak out. Furthermore, as the compressor is operating, the compression zones are heated more than the rest of the casing. The combination of this localized heating and the thickness variations in the casing results in non-uniform thermal expansion causing tight clearances between the rotating elements. In a conventional machine, the structural shifting due to pressure must be accepted or the structure made heavier. When the structure is made heavier, a weight penalty results. That is, as the inlet pressure increases, the mechanism requires heavier structural components, making the mechanism progressively more difficult to build, install and maintain, or if the structural strength is not increased, the leakage due to the shifting of the rotational axes under the stress of inlet pressure results in a decrease in the efficiency of the compressor. Unfortunately, as the structural components are made heavier to avoid the structural shifting due to increased inlet pressure, the reduction of clearances in a conventional machine due to the localized thermal expansion are aggravated.
Consequently, there is a need for a single screw mechanism that avoid the rotational axes shifts that result from a pressurized inlet or from internal thermal expansion.